The present invention concerns the adsorption arts. More particularly, the present invention concerns high-performance adsorbents based on activated carbon of high meso- and macroporosity and a process for production thereof and also the use of these high-performance adsorbents, particularly for adsorptive filtering materials, for the food industry (for example for preparing and/or decolorizing food products, for the adsorption of toxins, noxiants and odors, particularly from gas or air streams, for purifying or cleaning gases, particularly air, and liquids, particularly water, for application in medicine or to be more precise pharmacy, and also as sorptive storage media particularly for gases, liquids and the like.
Activated carbon has fairly unspecific adsorptive properties and therefore is the most widely used adsorbent. Legislation as well as the rising sense of responsibility for the environment lead to a rising demand for activated carbon.
Activated carbon is generally obtained by carbonization (also referred to by the synonyms of smoldering, pyrolysis, burn-out, etc) and subsequent activation of carbonaceous compounds, preferably such compounds as lead to economically reasonable yields. This is because the weight losses through detachment of volatile constituents in the course of carbonization and through the subsequent burn-out in the course of activation are appreciable. For further details concerning the production of activated carbon, see for example H.v. Kienle and E. Bäder, Aktivkohle and ihre industrielle Anwendung, Enke Verlag Stuttgart, 1980.
The constitution of the activated carbon produced—finely or coarsely porous, firm or brittle, etc—depends on the starting material. Customary starting materials are coconut shells, charcoal and wood (for example wood wastes), peat, bituminous coal, pitches, but also particular plastics which play a certain part in the production of woven activated carbon fabrics for example.
Activated carbon is used in various forms: pulverized carbon, splint coal carbon, granulocarbon, molded carbon and also, since the end of the 1970s, spherical activated carbon (“spherocarbon”). Spherical activated carbon has a number of advantages over other forms of activated carbon such as pulverized carbon, splint coal carbon, granulocarbon, molded carbon and the like that make it useful or even indispensable for certain applications: it is free flowing, abrasion resistant or to be more precise dustless, and hard. Spherocarbon is in great demand for particular applications, for example, because of its specific form, but also because of its high abrasion resistance.
Spherocarbon is mostly still being produced today by multistage and very costly and inconvenient processes. The best known process consists in producing spherules from bituminous coal tar pitch and suitable asphaltic residues from the petrochemical industry, which are oxidized to render them unmeltable and then smoldered and activated. For example, spherocarbon can also be produced in a multistage process proceeding from bitumen. These multistage processes are very cost intensive and the associated high cost of this spherocarbon prevents many applications wherein spherocarbon ought to be preferable by virtue of its properties.
WO 98/07655 A1 describes a process for producing activated carbon spherules wherein a mixture comprising a diisocyanate production distillation residue, a carbonaceous processing aid and if appropriate one or more further additives is processed into free-flowing spherules and subsequently the spherules obtained in this way are carbonized and then activated.
It is further prior art to produce spherocarbon by smoldering and subsequent activation of new or used ion exchangers comprising sulfonic acid groups, or by smoldering ion exchanger precursors in the presence of sulfuric acid and subsequent activation, the sulfonic acid groups and the sulfuric acid respectively having the function of a crosslinker. Such processes are described for example in DE 43 28 219 A1 and DE 43 04 026 A1 and also in DE 196 00 237 A1 including the German patent-of-addition application DE 196 25 069 A1.
However, there are a number of specific applications where it is not only the geometry or to be more precise the external shape of the activated carbon which is of decisive importance, but also its porosity, in particular the total pore volume and the adsorption capacity on the one hand and the distribution of the pores, i.e., the fraction of micro-, meso- and macropores in relation to the total pore volume, on the other.
There are a number of applications requiring a particularly high meso- and macroporosity of the activated carbon, i.e., a high meso- and macropore volume fraction, coupled with an altogether high total pore volume, for example in relation to the applications mentioned at the beginning, for example for use in the food industry, in the manufacture of certain adsorptive filtering materials (for example for NBC protective apparel), for the adsorption of toxins, noxiants and odors, particularly from gas or air streams, for purifying or cleaning gases, such as in particular air, and also liquids, for application in medicine or to be more precise pharmacy, in the sorptive storage of gases or liquids and the like.
True, the activated carbon known for this purpose from the prior art does have a certain degree of meso- and macroporosity, but that degree is not sufficient in all cases. In addition, increasing porosity is often observed to be accompanied by an unwelcome, occasionally unacceptable decrease in mechanical stability or to be more precise abrasion resistance. Nor are the fraction of the total pore volume which is accounted for by meso- and macropores and the absolute pore volume always sufficient to ensure adequate performance capability and/or an adequate impregnatability (for example impregnation with metals or metal salts) for all applications.